The present invention relates to a power transmission system for vehicles, and more particularly to a power transmission system suitable for use in a four-wheel drive motorcar in which front wheels and rear wheels are both driven.
In motorcars, normally, power of an engine is transmitted to left and right drive wheels via a differential gear unit, and this differential gear unit is complexed in structure and heavy in weight. Moreover, since if either one of left and right drive wheels becomes a no-load condition it occurs that a torque cannot be transmitted to either drive wheel, in order to avoid such condition a differential operation limiter must be provided, and consequently, the differential gear unit becomes further complexed and has its weight increased.
As a replacement for such a differential gear unit, differential drive means making use of a viscous fluid coupling mechanism has been proposed, for instance, in Laid-Open Japanese Patent Publication No. 58-50349 (1983). In this differential drive means, viscous fluid such as silicone fluid or the like is enclosed within a housing which is connected to a drive shaft on an engine side and adapted to be rotated jointly with that drive shaft, and left and right axles are fitted into the same housing. In addition, within the same housing are assembled a large number of annular plates which are mounted respectively to that housing and the left and right axles opposed to each other, so that rotation of the housing can be transmitted to the left and right axles by the intermediary of a viscous shearing force of the viscous fluid. Furthermore, owing to fluid slip of that viscous fluid, the left and right axles can operate differentially with respect to each other and with respect to the housing.
In the above-referred Laid-Open Japanese Patent Publication No. 58-50349 is also disclosed a power transmission system in which power of an engine is directly transmitted to front wheels through the conventional differential gear unit, while it is transmitted to rear wheels via differential drive means making use of a viscous fluid coupling mechanism as described above.
As described above, a power transmission system making use of a differential drive unit of viscous fluid coupling type becomes simple in structure and has its weight reduced, but in such a differential drive unit it may possibly happen that the above-described annular plates themselves take a mutually locked condition and hence relative slip therebetween is prevented. If such a condition should arise, transmission of an excessively large torque would become possible because the housing and the axles are directly coupled via the annular plates, and accordingly, mechanical strengths of the respective parts in the drive system must be made so strong as being compatible to such an excessively large torque, so that one cannot expect to achieve desired reduction of the weight of the unit.